With increase in the operating temperature of a CCD imager, the dark current generated in the semiconductive substrate by thermal excitation increases. Each charge packet is descriptive of the energy of an image element. During its passage across the substrate through successively induced potential energy wells, the charge packet is augmented by dark current charge collection in those wells. So when these charge packets are finally sensed, dark-current components undesirably accompany the photoconversion response components. These dark-current components can be reduced in the imager output signal by cooling the imager or by subtracting away the dark-current components. If the imager cooling is regulated, or if dark current components are to be suppressed by a subtracting away process, it is necessary to derive a temperature-dependent or dark-current-dependent signal.
To provide best tracking between this signal and the dark current components accompanying photoresponse components, it is desirable to develop this signal as a dark-current dependent signal, deriving it from collected dark current. The collection of dark current is preferably from a significantly large portion of the semiconductor die so that the integrated dark current is large compared to its accompanying noise. This noise arises because collection of dark current is a statistical, quantized process. E.g., there can be a collection of remnant charge transferred out of the field storage (B) register of a CCD image of field transfer type during field transfer times and transmitted through the output line (C) register to the output electrometer stage. But while this general scheme works for CCD imagers of field transfer or interline transfer type, this does not work for other types of CCD imagers, such as the line transfer type. Furthermore, there is usually some degree of light leakage into the field storage register which generates spurious photoresponse that can undesirably augment low-level dark-current components. Low-level dark-currents can also be undesirably augmented by noise from the electrometer.
A potential energy well can be induced in the surface of a portion of the semiconductor die shielded from irradiation, which well is relatively large in area compared to the potential wells in the CCD registers of the imager itself. Dark current collected in this well can be sensed with an electrometer to generate a dark-current dependent voltage that is relatively noise-free. This permits dark-current field shading cancellation to be carried out without appreciably reducing the signal-to-noise ratio of the output video signal. Since the dark-current-dependent signals are slow-changing, the field effect transistor used in a floating diffusion electrometer output stage can be operated at lowered current levels to reduce heating. Furthermore, the electrometer transistor can be made a small-area device which reduces gate capacitance and so in accordance with Coulomb's Law increases electrometer sensitivity to charge.